The primary goal of this research is a greater understanding of the seasonal, annual, and interannual mean state and variability of water and energy cycles at continental-to-global scales, and thus a greater understanding of the interactions among the terrestrial, atmospheric, and oceanic hydrosphere in the Earth’s climate system.

    This understanding will be achieved through a combination of observations, modeling, and analysis at a range of spatial and temporal scales, and will provide the foundations for understanding the relationship between weather (the manifestation of fast atmospheric hydrologic processes) and climate (the long-term statistical measures of these hydrological processes.) The research program aims at furthering our understanding of these relationships — especially the relationship between the physical representation of fast hydrologic processes and climatic statistics; the relative roles of land, atmosphere, and ocean hydrologic processes in weather and climate at continental-to-global scales, from daily to interannual timescales; and a determination of how these relationships and roles vary globally and seasonally. Such advances should lead to improved inferences about the occurrence of severe weather events, such as floods and drought, that directly affect property and human safety, and permit the downscaling of hydrological variables (precipitation, surface meteorology, etc.) that can lead to improved water and environmental management.

    An important element of the research program is a quantitative assessment of the improved understanding for weather prediction and for water and environmental management. In addition, advances in understanding the relationships between hydrologic processes and climate will lead directly to better inferences regarding climate change and its subsequent hydrologic impacts at regional-to-global scales. Improving this understanding is hampered by the complexity of the nonlinear hydrologic processes, and in the heterogeneity related to both process forcings and process parameters that exists at all spatial and temporal scales. Understanding is also hampered by a lack of consistent, systematic observations, making it difficult to develop and test new theories and hypotheses regarding the global water cycle.

Key research challenges include:
1. Land Surface Interactions: Developing a better understanding of the coupling of land surface hydrologic processes to atmospheric processes over a range of spatial and temporal scales; the role of the land surface in climate variability and climatic extremes; and the role of the land surface in climate change and terrestrial productivity.
2. Atmospheric Processes: Developing a better understanding of the role of clouds and their influence in the coupling of the atmospheric water and energy cycles, and of the vertical transport and mixing of water vapor on scales ranging from the local boundary layer to regional weather systems.

Focus for FY 2000:
• The USGCRP will demonstrate skill in predicting changes in water resources and soil moisture on timescales up to seasonal and annual as an integral part of the climate system. As a first step, the program will quantify evaporation, precipitation, and other hydrological processes as required to improve prediction of regional precipitation over periods of one to several months
• The USGCRP will demonstrate the ability to determine radiative fluxes and diabatic heating within the atmosphere and at the surface with the precision needed to predict transient climate variations and to understand natural and anthropogenically-forced climate trends.
• The USGCRP will combine Tropical Rainfall Measuring Mission (TRMM) measurements with rainfall measurements from other sources to set a benchmark for rainfall in the tropics. We will obtain maps of the diurnal cycle of precipitation (which cannot be obtained from sun-synchronous sensors). The insight gained from this exercise will be used to reprocess 10 years of SSM/I data for climate record. This 10-year data set and ongoing TRMM measurements will be used to validate climate models as well as demonstrate the impact of rainfall in assimilation and weather forecast schemes.
• The USGCRP will establish a climatologically valid database of 60 months of rainfall data from various ground validation radar sites. The program will achieve 10% agreement among the various TRMM-related sensors for zonally averaged monthly rainfall accumulations. This will establish our confidence in how well tropical rainfall, a central component of the global water cycle, can be measured from space.
• The USGCRP will complete cloud model simulations of major storm systems in the Brazilian Amazon and at the Kwajalein atoll oceanic site for the purpose of testing latent heating estimates from TRMM.
• The USGCRP will assess the accuracy of remote and in-situ humidity measurements, and improve understanding of the climate consequences of water vapor radiation feedback. The program will conduct a field experiment at the DOE radiation testbed facility in Oklahoma, under joint NASA and DOE sponsorship.
• The USGCRP will conduct data comparison workshops, establish validation sites, and expand and improve global water vapor data sets toward the goal of quantifying and understanding the role of water vapor in meteorological, hydrological, and climatological processes.
• The USGCRP will examine linkages between land-atmosphere processes, their relationship to anthropogenic and other emissions, and the consequences of their deposition to the functioning of the biogeophysical and biogeochemical systems of southern Africa. This initiative is being built around a number of ongoing activities supported by the U.S., the international community, and African nations in the southern African region.